When a cover or the like, e.g. a lid, foil seal or the like, is not seated correctly on the container, or the traces of product which are packaged is left between the surfaces of the cover or container to be sealed together, by a pressed-fit seal, an adhesive seal or a weld, e.g. an ultrasonic or heat-seal weld, the defective seal can result in spoilage of the contents of the container. This also applies to cases in which the rim of the container or the cover may be damaged, either on manufacture or in the container filling and closing process.
In all such cases, the filled container cannot be closed hermetically or in an air-tight manner so that the spoilage can occur. The danger is especially prominent in cases in which the product filled into the container is a comestible or foodstuff, for example, a dairy product like yogurt, cottage cheese or the like, where absolutely tight seals are essential if the product is not to become rapidly unusable.
As a consequence, systems have been developed for testing the tightness of the seal of such packages, generally in association with a filling and container-closing line or machine.
In German open application DE-OS 37 25 739, the tightness of the seal of plastic cups containing a packaged product and having a cover welded onto the cup can be tested by utilizing the bulge of the cover as a criterium. The bulge of the cover is electronically measured and the electronic measurement is evaluated by a computer.
When the cup is subjected to a subatmospheric pressure, the bulge of the cover develops a maximum which can be measured, the measurement digitalized and the digital signal subtracted in the computer from a reference value, the difference being compared with a specific preselected magnitude. A voltage change on a contactless analog displacement measuring sensor thus can be used to signal the state of the tested product. In the case of a container which is not sufficiently sealed or tight, the signal results in a sorting out of the defective container downstream of the testing station. This testing method requires, as a consequence of the electronic sensors used, costly apparatus.
U.S. Pat. No. 4,934,180 discloses a seal testing unit which seals a pressure chamber against the container and allows a pressure medium to be introduced into the annular pressure chamber so that the pressure medium can penetrate into possible openings in the weld seam between the cover and the container to cause the cover to bulge outwardly. A signal is triggered by this bulge.
Instead of introducing a fluid under pressure to the container, a subatmospheric pressure can be applied, and in this case the cover will remain in a lower position. In either technique the lack of tightness of the container is established and the defective container can be sorted out. This system utilizes a mechanical sensing of the displacement of the cover which follows the cover and requires a certain stability of the shape of the cover.
Another drawback of this arrangement is that it allows only the sealing zone of the cover and container to be tested and does not respond to a defect in the cover itself except at the sealing zone.
German open application DE-OS 37 18 600 discloses a method of and an apparatus for determining the tightness of filled containers having a cover or the like sealed or welded thereto by squeezing the container together to impart a bulge to the cover in the case of a sealed container. The height of the bulge is measured as an indication of the tightness of the cover utilizing a contact sensor.
Apart from the fact that this system is sensitive to the characteristics and shape stability of the container and/or its cover, whereby different heights of the bulge can be generated with different degrees of application of the squeezing forces, this system also creates the possibility that the container will be excessively squeezed and damaged even if the seal was originally sufficient. Furthermore, repeated adjustment of the sensor is required with this arrangement.
German Patent 35 28 248 discloses a method of and an apparatus for monitoring the tightness of filled containers in which between the cover or the like and the product filled into the container, an air or gas cushion is provided.
This gas cushion is heated from above the cover and the cover is pressed downwardly by mechanical means. The position of the cover is detected and from this position it is determined whether the cover or the seal is intact or defective. This system operates satisfactorily but requires exact matching to the cover used. It is also important that the heating heat not damage any imprint provided on the cover or distort the cover thermally. Furthermore, for different materials from which the cover is made, efforts must be taken to ensure as uniform a thermal conductivity as is possible.